A communication network that uses optical fibers is constructed in a core network that connects together metropolises and a metro network that connects together bases in an area. In such a network, a plurality of optical fibers are used in a bundle. Wavelength division multiplexing (WDM) transmission which involves multiplexing a plurality of optical signals having different wavelengths is performed on respective individual optical fibers to realize high-capacity signal transmission (for example, see Non-Patent Literature 1). In order to further increase the transmission capacity, the use of a multi-core fiber (MCF) which is an optical fiber having a plurality of cores instead of an optical fiber (single core fiber: SCF) having one core has been discussed (for example, see Non-Patent Literatures 2 and 3).
In a network configured using an MCF, since a transmission volume in one MCF is large, a fault in an MCF may have an influence on communication between a plurality of nodes. Due to this, when a fault occurs in an MCF, it is necessary to quickly detect the fault and specify a faulty MCF.